A HARQ indicator is used in mobile communications to indicate a packet acknowledgment status (ACK or NACK) based on the reception and decoding of a corresponding packet. An ACK informs the transmitting end (UE or base station) that a retransmission is unnecessary, and a NACK indicates that retransmission is necessary. This improves the integrity of the transmitted data by ensuring that missing or corrupted packets are resent.
Typically, when a HARQ indicator is received by a UE, a determination is made whether the HARQ indicator is an ACK or a NACK based on the sum of the in-phase and the quadrature components of the received signal. If the sum is above a detection threshold, the HARQ indicator is an NACK, if the sum is below the detection threshold, the HARQ indicator is a ACK.
There are two situations in which an error may occur in detecting whether the HARQ indicator is an ACK or a NACK—an ACK may be incorrectly recognised as a NACK or a NACK may be incorrectly recognised as an ACK. The first case results in unnecessary retransmission, wasting a small amount of radio resources. However, the second case is more serious, resulting in loss of the packet, as it is not retransmitted. The 3rd-Generation Partnership Project (3GPP) sets targets of error detection rate for ACK→NACK and for NACK→ACK of P(ACK→NACK)=0.1% and P(NACK→ACK)=1%.
These targets may be difficult to achieve when there is a change in power level on the HARQ channel. The power level may change because of a change in distance between the base station and UE, switching to a different base station or disturbances on the path between the base station and UE.
This change in power level results in a change in the detection threshold used to determine whether a HARQ indicator is an ACK or NACK. The UE is not informed of these changes, and therefore may incorrectly decode a HARQ indicator.
It would be desirable to provide a method of determining whether a HARQ indicator is an ACK or a NACK, which is independent of the power level of the HARQ channel.